VNC and two firewall

Hello,
In a LAN I need to connect remotly with VNC to a client computer with Windows OS. The problem is that there are two firewalls or better one is a modem/router zyxel and then there is firewall on a server Linux (that it is used as backup file and mail server and other services) and after that there are the PC clients of the LAN:
INTERNET (WAN) -- ZYXEL/FIREWALL -- LINUX FIREWALL -- PC LAN
About the modem/router Zyxel I noticed that there are sets called "SUA" where you can set the IP and the port where to connect VNC. But the problems is with Linux in a generic LAN. Where I have a Linux machine connect to Internet I have these rules:
iptables -t nat -A PREROUTING -i ippp0 -p tcp -m tcp --dport 5900 -j DNAT --to-destination 192.168.0.2:5900
iptables -A FORWARD -i ippp0 -p tcp --dport 5900 -m state --state NEW,ESTABLISHED -j ACCEPT

so everything works. Do you think it is enought to leave open the ports of the Firework Zyxel (5900) and then to set the rules with iptables on Linux?

Thanks a lot!
Take care